This invention relates to a biological process for the purification of waste air streams laden with sparingly water-soluble substances of which the biological degradation is accompanied by the formation of acids.
The biowasher process is known from the literature. In this process, the organic impurities are washed out from the waste air stream in an absorber and the washing liquid is biologically purified in an activated sludge tank. The purified liquid is then recirculated into the absorber and re-charged. However, the uptake capacity of the liquid for the substances mentioned above is so small that the recirculation stream between the two units becomes so large that the process cannot be used for economic reasons.
U.S. Pat. No. 0 249 861 A2 describes a process in which the absorber and the activated sludge reactor are integrated into a a single apparatus, a plate column. Compared with the spray or jet washer, the scope of application is distinctly broader. However, since the necessary number of stages and hence the pressure loss decrease with decreasing solubility of the waste air ingredients in water, there are also limits to this process.
In another biowasher process (DE 3 641 442 A1), silicone oil is added to the washing liquid so that the liquid is able to absorb more organic substances. However, removal of the salts formed is problematical. In addition, silicone oil is always discharged with the salt and has to be replaced.
The biofilter process is unsuitable for the elimination of substances of which the degradation is accompanied by the formation of HCl or H.sub.2 SO.sub.4 because the biofilm acidifies so that the degradation activity of the microorganisms comes to a standstill.
The trickling filter process is mentioned in VDI Richtlinie 3478. In contrast to the biowasher process, the microorganisms are immobilized in the trickling filter process. The gas is passed through a packing in which the mass transfer of the organic constituents and the oxygen takes place. In addition, a liquid stream is passed over the packing, neutralizing the H.sup.+ ions, removing the salt accumulating and supplying the bacteria with nutrient salts. Through the immobilization of the microorganisms, salts can be removed without any loss of bacteria. Hitherto, trickling filters have mainly been used for odor elimination.
Neutralization of the acids formed during the degradation of A and Cl compounds is difficult on account of the limited pH range in which microorganisms are viable and the uneven flow through packings. If too little liquid is distributed through the packing, the biofilm acidifies and the degradation activity of the organisms is reduced. However, if the sprinkling density is too high, the liquid preferentially blocks channels in which there are large quantities of biomass. These organisms then take no further part in elimination of the waste air ingredients so that degradation performance deteriorates.